Shielding design of particle therapy facilities

Full text: This paper provides an overview of the shielding design of particle therapy (PT) facilities. At these facilities protons and various ions such as helium, lithium, carbon, oxygen and neon are used for radiation therapy. There are currently about thirty operational particle therapy facilities worldwide. Another twenty facilities or so are in the planning, design or construction stage. A typical PT facility consists of an injector, a cyclotron or a synchrotron, a high energy transport beam line, several treatment rooms including fixed beam and gantry rooms, and even a research area. During the operation of these facilities, radiation is produced with neutrons being the dominant component outside the shielding. Beam loss considerations for cyclotron based facilities are different from synchrotron facilities. Cyclotrons are fixed energy machines and use energy degraders to reduce the energy of the particle, resulting in the production of neutrons and activation. Synchrotrons on the other hand are designed to accelerate protons and ions to the exact energy needed for therapy, thus eliminating the need for energy degraders. This in turn results in less local shielding and activation of beam-line components. At these facilities proton energies typically range from about 230 to 250 MeV, while carbon ions may have energies up to a maximum of 320 MeV/u to 430 MeV/u. For the carbon beams, the neutron spectrum extends approximately to 2.5 times the energy of the carbon ion. For proton beams, the neutron energies extend to a maximum which is the energy of the incident proton. The neutron dose equivalent from carbon ions dominates the shielding in the forward direction. However, at large angles the total neutron dose equivalent from protons dominates the shielding because the proton intensities are about 25-40 times higher than the carbon ion intensities. Several vendors provide turnkey designs. The pitfalls of using cookie cutter shielding designs are pointed out. The importance of considering the patient workload, the beam parameters for treatment, beam losses, appropriate targets, the country/state specific regulatory requirements, the occupancies in the adjacent areas and contribution from multiple sources is stressed. The angular dose equivalent profile for protons and carbon ions of various energies incident on various targets, spectra, and transmission of various shielding materials obtained by Monte Carlo calculations (methodology is discussed in a previous paper) are presented. (author)